Coking processes have been practiced for many years and are an important source of revenue for many refineries. In a coking process, heavy hydrocarbon feedstock is thermally decomposed, or cracked, into coke and lighter hydrocarbon products. Of the various types of coking processes currently used in the petroleum refining industry, delayed coking has emerged as the technology of choice by most refiners due to its lower investment costs and its ability to produce comparable yields of products but of higher quality.
A typical delayed coking process is a semi-continuous process in which heavy hydrocarbon feedstock is heated to cracking temperature using a heat source such as a coker furnace. The heated feedstock is then fed continuously to a coking drum, where it reacts in its contained heat to convert the feedstock to coke and cracked vapors. The cracked vapors are passed overhead to a coker fractionator, condensed and recovered as lower boiling hydrocarbon products. The fractionator bottoms may be recycled to the feedstock if desired. When the coke drum contents reach a predetermined level, the feedstock supply is switched to another drum, and the full drum is cooled and de-coked. The entire process for one drum, from fill cycle start to fill cycle start, may require between 18 and 120 hours.
Depending upon system design, operating parameters and feedstock, delayed coking is capable of producing a range of coke grades having differing physical properties. Coke properties determine its use and economic value. A high quality grade of coke, needle coke, is a primary constituent of graphite electrodes used in electric arc furnaces employed in the steel industry. Needle coke is produced from low asphaltenic, highly aromatic, low metal and low sulfur feedstock and is characterized as having a low coefficient of thermal expansion (“CTE”) and high density. Even small changes in coke CTE and density can have substantial effects on electrode properties. An intermediate quality grade of coke, anode coke, is used primarily for the production of anodes employed in aluminum manufacture. Anode coke, which has technical specifications and economic value that fall between those of needle coke and fuel coke, is produced from low sulfur and relatively low metal feedstock. While CTE is not a factor in the characterization of anode coke, higher coke density is desirable for such coke. The term “premium” is sometimes used to refer to needle coke, but because needle coke and anode coke have higher economic value than fuel coke, the term is also used, depending upon context, to refer to any coke having one or more qualities which make it superior to fuel coke. Fuel coke is used primarily for fuel for power stations and cement kilns. Fuel coke, which has the lowest economic value, is produced from high sulfur, high metal feedstock.
In a delayed coking process, feedstock is introduced to the coking drum during the entire fill cycle. If the fill cycle lasts for 30 hours, the feedstock first introduced to the coking drum is subjected to coking conditions for that 30 hour period of time. Each succeeding increment of feedstock, however, is coked for a lesser period of time and the final portion of feedstock introduced to the coking drum is subjected to coking conditions only for a relatively short period of time. In view of this, problems can be encountered in obtaining coke product having consistent properties throughout the drum. Coke produced near the top of the drum, where reaction times are short, generally has different physical properties than coke produced in the remainder of the drum. Unconverted feedstock in the coking drum at the end of the coking process can result in the formation of coke that is high in volatile matter. However, coke having varying levels of volatile matter can be found throughout a coke drum, suggesting that coke strength, porosity and particle size, are not consistent throughout the drum. Coke which is not consistent in properties throughout the drum presents problems in production of both electrodes for the steel industry and anodes for the aluminum industry. Such inconsistency can lead to poor electrode performance and/or premature cracking of the electrode.
In the production of coke, there are competing interests. High coking temperatures increase reaction rates and shorten reaction times, but decrease coke yield. Moreover, at a certain point, increased temperatures result in coke having higher CTE values. Low coking temperatures, in contrast, normally result in slower reaction rates and longer reaction times, but increase coke yield and produce coke having lower CTE values. Pressure, fill rate, and recycle ratio also affect coke yield and quality. It is necessary, therefore, to reach an acceptable point between low quality/high quantity coke production and high quality/low quantity coke production which provides the greatest amount of coke meeting industry quality specifications. In the manufacture of needle coke, it is known for example to carry out the coking reaction at lower coking temperatures and, after the drum is filled and feedstock introduction has ceased, to heat treat the resulting coke by contacting it with a non-coke forming material which is in the vapor state at a higher temperature than the coking temperature. This type of operation is undesirable due to the formation of a low density “fluff” material during the switch to the non-coke forming vapors. The problem of fluff formation has been addressed by carrying out the coking reaction at lower coking temperatures and, after the drum is filled and feedstock introduction has ceased, to heat treat the resulting coke by contacting it with an admixture of an aromatic mineral oil capable of forming coke and a non-coking material at a temperature equal to or higher than the coking temperature and optionally thereafter further heat treating the coke by contacting it with a non-coking material at a temperature higher than the coking temperature. Although this type of operation reduces fluff formation, it suffers from the drawbacks of the additional processing complexity associated with the use of the admixture and the additional processing time associated with the heat treatment steps.
It would be advantageous to provide a delayed coking process which can produce coke having improved physical properties and/or produce coke having more consistent physical properties throughout the coke drum. It would also be desirable to provide a simple and cost effective process that can increase the coke production capacity of existing coking facilities by, for example, decreasing, if not eliminating, the need to use a heat treatment step.